docs: fault-injection: document cache-filter feature for failslab

The failslab fault injection mechanism has an undocumented capability
that provides significant utility in testing and debugging. This feature,
introduced in commit 4c13dd3b ("failslab: add ability to filter slab
caches"), allows for targeted error injection into specific slab caches.

However, it was inadvertently left undocumented at the time of its
implementation.

Add documentation for the cache-filter feature in the failslab mode
description. Also, providing a practical example demonstrating how to
use cache-filter to inject failures specifically when allocating socket
buffers (skbs).
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240726120930.3231333-1-leitao@debian.org

Documentation/fault-injection/fault-injection.rst

@@ -141,6 +141,14 @@ configuration of fault-injection capabilities.
 	default is 'Y', setting it to 'N' will also inject failures into
 	highmem/user allocations (__GFP_HIGHMEM allocations).
 
+- /sys/kernel/debug/failslab/cache-filter
+	Format: { 'Y' | 'N' }
+
+        default is 'N', setting it to 'Y' will only inject failures when
+        objects are requests from certain caches.
+
+        Select the cache by writing '1' to /sys/kernel/slab/<cache>/failslab:
+
 - /sys/kernel/debug/failslab/ignore-gfp-wait:
 - /sys/kernel/debug/fail_page_alloc/ignore-gfp-wait:
 
@@ -459,6 +467,18 @@ Application Examples
     losetup -d $DEVICE
     rm testfile.img
 
+------------------------------------------------------------------------------
+
+- Inject only skbuff allocation failures ::
+
+    # mark skbuff_head_cache as faulty
+    echo 1 > /sys/kernel/slab/skbuff_head_cache/failslab
+    # Turn on cache filter (off by default)
+    echo 1 > /sys/kernel/debug/failslab/cache-filter
+    # Turn on fault injection
+    echo 1 > /sys/kernel/debug/failslab/times
+    echo 1 > /sys/kernel/debug/failslab/probability
+
 
 Tool to run command with failslab or fail_page_alloc
 ----------------------------------------------------